Acrylic rubber, rubber composition, and crosslinked product

Abstract

The acrylic rubber contains, as monomer units, a (meth) acrylic acid ester, a carboxylic acid vinyl ester, and ethylene, the carboxylic acid vinyl ester content being 15% by mass or more and the ethylene content being 5% by mass or more based on the total amount of monomer units in the acrylic rubber.

Description

Technical Field

[0001] The present disclosure relates to acrylic rubber, a rubber composition and a cross-linked product thereof. Background Art

[0002] Crosslinked acrylic rubbers are used as materials such as hoses and sealing parts in the engine compartment of automobiles because of their excellent physical properties such as heat resistance, oil resistance, and mechanical properties. For example, Patent Document 1 discloses an acrylic rubber composition having excellent heat resistance, particularly elongation at break and residual elongation after heat aging without impairing normal physical properties, which is obtained by copolymerizing 100 parts by mass of an alkyl (meth)acrylate, 1 to 10 parts by mass of vinyl acetate, and 1 to 3 parts by mass of a crosslinking site monomer.

[0003] Prior art literature

[0004] Patent Literature

[0005] Patent Document 1: Japanese Patent Application Publication No. 2009-91437 Summary of the invention

[0006] According to the research conducted by the present inventors, it has been found that when the tensile strength, which is one of the mechanical properties (normal physical properties) of a cross-linked acrylic rubber, is increased, the oil resistance of the cross-linked acrylic rubber may be deteriorated.

[0007] Therefore, an object of one aspect of the present invention is to provide an acrylic rubber which can provide a cross-linked product having high tensile strength and excellent oil resistance.

[0008] The present inventors have found that, in an acrylic rubber comprising (meth)acrylate, vinyl carboxylate and ethylene as monomer units, when the vinyl carboxylate content and the ethylene content are respectively not less than predetermined contents, an acrylic rubber having high tensile strength and excellent oil resistance can be provided.

[0009] The present invention includes the following aspects.

[0010] [1] An acrylic rubber comprising (meth)acrylic acid ester, vinyl carboxylate and ethylene as monomer units, wherein the content of the vinyl carboxylate is 15% by mass or more and the content of ethylene is 5% by mass or more, based on the total amount of the monomer units in the acrylic rubber.

[0011] [2] The acrylic rubber according to [1], wherein the mass ratio of the ethylene content to the vinyl carboxylate content is 0.1 to 0.5.

[0012] [3] The acrylic rubber according to [1] or [2], wherein the (meth)acrylate contains at least one selected from the group consisting of alkyl (meth)acrylates and alkoxy (meth)acrylates.

[0013] [4] The acrylic rubber according to any one of [1] to [3], wherein the carboxylic acid vinyl ester includes at least one selected from the group consisting of vinyl acetate, vinyl neodecanoate, and vinyl laurate.

[0014] [5] A rubber composition comprising the acrylic rubber according to any one of [1] to [4] and a cross-linking agent.

[0015] [6] A cross-linked product of the rubber composition described in [5].

[0016] One aspect of the present invention is to provide an acrylic rubber which can provide a cross-linked product having high tensile strength and excellent oil resistance. DETAILED DESCRIPTION

[0017] Hereinafter, embodiments of the present invention will be described in detail, but the present invention is not limited to the embodiments.

[0018] One embodiment of the present invention is an acrylic rubber containing (meth)acrylate, vinyl carboxylate and ethylene as monomer units. In the present specification, "(meth)acrylate" is used as a concept including both acrylate and its corresponding methacrylate.

[0019] The (meth)acrylate preferably contains at least one selected from the group consisting of alkyl (meth)acrylates and alkoxy (meth)acrylates.

[0020] The alkyl group in the alkyl (meth)acrylate may be linear or branched. The alkyl group in the alkyl (meth)acrylate may have 1 or more and 16 or less carbon atoms.

[0021] The content of the alkyl (meth)acrylate may be 10% by mass or more, 20% by mass or more, 30% by mass or more, 40% by mass or more, or 50% by mass or more, and may be 90% by mass or less, 80% by mass or less, 70% by mass or less, or 60% by mass or less, based on the total amount of monomer units in the acrylic rubber.

[0022] The alkyl (meth)acrylate includes at least one selected from acrylate and methacrylate, and preferably includes acrylate.

[0023] The alkyl acrylate is represented by the following formula (1).

[0024]

[0025] In the formula, R1 It represents an alkyl group.

[0026] The alkyl group (R 1 ) can be straight chain or branched. 1 ) may have a carbon number of 1 or more and 16 or less. Specific examples of the alkyl acrylate include methyl acrylate, ethyl acrylate, n-propyl acrylate, isopropyl acrylate, n-butyl acrylate, isobutyl acrylate, n-pentyl acrylate, isopentyl acrylate, n-hexyl acrylate, 2-methylpentyl acrylate, n-octyl acrylate, 2-ethylhexyl acrylate, lauryl acrylate, hexadecyl acrylate, 1-adamantyl acrylate, cyclohexyl acrylate, and the like. These alkyl acrylates may be used alone or in combination of two or more.

[0027] The content of the alkyl acrylate can be 10% by mass or more, 20% by mass or more, 30% by mass or more, 40% by mass or more, or 50% by mass or more, and can be 90% by mass or less, 80% by mass or less, 70% by mass or less, or 60% by mass or less, based on the total amount of monomer units in the acrylic rubber.

[0028] From the viewpoint of further improving the oil resistance of the cross-linked acrylic rubber, the alkyl acrylate preferably contains an alkyl group (R 1 The alkyl acrylate may further include an alkyl group having 4 or more carbon atoms (R 1 An alkyl acrylate having an alkyl group having 4 or more carbon atoms (a second alkyl acrylate).

[0029] The number of carbon atoms of the alkyl group in the first alkyl acrylate may be 1 or more, 2 or less, or 2. The first alkyl acrylate is preferably ethyl acrylate. The number of carbon atoms of the alkyl group in the second alkyl acrylate may be 8 or less, 6 or less, or 5 or less, or 4. The second alkyl acrylate is preferably n-butyl acrylate.

[0030] The content of the first alkyl acrylate may be 10% by mass or more, 20% by mass or more, 30% by mass or more, 40% by mass or more, or 50% by mass or more, and may be 90% by mass or less, 80% by mass or less, 70% by mass or less, or 60% by mass or less, based on the total amount of monomer units in the acrylic rubber.

[0031] The content of the second alkyl acrylate may be 5 mass % or more, 10 mass % or more, or 20 mass % or more, and may be 60 mass % or less, 50 mass % or less, or 40 mass % or less, based on the total amount of monomer units in the acrylic rubber.

[0032] The alkyl methacrylate is represented by the following formula (2).

[0033]

[0034] In the formula, R 2 It represents an alkyl group.

[0035] The alkyl group (R 2 ) can be straight chain or branched. The alkyl group (R 2 ) may have a carbon number of 1 or more, or 4 or less, preferably 2 or more or 3 or more, and may also be 3. Specific examples of alkyl methacrylates include methyl methacrylate, ethyl methacrylate, n-propyl methacrylate, isopropyl methacrylate, n-butyl methacrylate, isobutyl methacrylate, and the like. These alkyl methacrylates may be used alone or in combination of two or more. From the viewpoint of improving the heat resistance of acrylic rubber, the alkyl methacrylate is preferably n-butyl methacrylate.

[0036] The content of the alkyl methacrylate may be 5 mass % or more, 7 mass % or more, or 10 mass % or more, and may be 30 mass % or less, 20 mass % or less, or 15 mass % or less, based on the total amount of monomer units in the acrylic rubber.

[0037] The alkoxyalkyl (meth)acrylate is represented by the following formula (3).

[0038]

[0039] In the formula, R 3 represents a hydrogen atom or a methyl group, R 3 represents an alkylene group, R 4 It represents an alkyl group.

[0040] The alkylene group (R 4 ) and alkyl (R 5 ) may be straight chain or branched. The alkylene group (R 4 ) may have 1 or more, or 2 or more, or 4 or less, or 3 or less. The alkyl group (R 5 ) may have 1 or more carbon atoms, 4 or less, 3 or less, or 2 or less.

[0041] Specific examples of the alkoxyalkyl (meth)acrylate include 2-methoxyethyl (meth)acrylate, 2-ethoxyethyl (meth)acrylate, 2-n-propoxyethyl (meth)acrylate, 2-n-butoxyethyl (meth)acrylate, 3-methoxypropyl (meth)acrylate, 3-ethoxypropyl (meth)acrylate, 2-n-propoxypropyl (meth)acrylate, and 2-n-butoxypropyl acrylate. These alkoxyalkyl (meth)acrylates may be used alone or in combination of two or more.

[0042] The content of the alkoxyalkyl (meth)acrylate may be 5 mass % or more, 10 mass % or more, or 15 mass % or more, and may be 30 mass % or less, 25 mass % or less, or 20 mass % or less, based on the total amount of monomer units in the acrylic rubber.

[0043] The carboxylic acid vinyl ester is preferably a carboxylic acid vinyl ester (vinyl ester of aliphatic monocarboxylic acid) represented by the following formula (4).

[0044]

[0045] In the formula, R 6 It represents an alkyl group.

[0046] R 6 The alkyl group represented by R may be linear or branched. 6 The carbon number of the alkyl group represented by may be 1 or more, 12 or less, 11 or less, 10 or less, or 9 or less. Specific examples of vinyl carboxylates include vinyl acetate, vinyl neodecanoate, vinyl laurate, vinyl propionate, vinyl butyrate, vinyl caproate, vinyl octanoate, vinyl decanoate, vinyl octanoate, and vinyl pivalate. The vinyl carboxylate preferably contains at least one selected from vinyl acetate, vinyl neodecanoate, and vinyl laurate, and more preferably contains vinyl acetate.

[0047] The content of the vinyl carboxylate is 15% by mass or more from the viewpoint of obtaining a cross-linked product having high tensile strength and excellent oil resistance, based on the total amount of monomer units in the acrylic rubber. From the viewpoint of more easily obtaining the cross-linked product, it may be preferably 17% by mass or more, 20% by mass or more, 25% by mass or more, 30% by mass or more, or 35% by mass or more, and may be 80% by mass or less, 75% by mass or less, 70% by mass or less, 60% by mass or less, 50% by mass or less, or 40% by mass or less.

[0048] The ethylene content is 5% by mass or more from the viewpoint of obtaining a cross-linked product having high tensile strength and excellent oil resistance, based on the total amount of monomer units in the acrylic rubber. From the viewpoint of more easily obtaining the cross-linked product, the ethylene content is preferably 6% by mass or more, 7% by mass or more, 8% by mass or more, 9% by mass or more, or 10% by mass or more, and may be 20% by mass or less, 17% by mass or less, or 15% by mass or less.

[0049] From the viewpoint of more easily obtaining a cross-linked product having high tensile strength and excellent oil resistance, the mass ratio of the content of ethylene to the content of vinyl carboxylate (content of ethylene / content of vinyl carboxylate) is preferably 0.1 to 0.5. The lower limit of the mass ratio may be 0.12, 0.14, 0.16, or 0.18. The upper limit of the mass ratio may be 0.4, 0.3, 0.25, 0.23, or 0.21.

[0050] The acrylic rubber may further contain a crosslinking site monomer as a monomer unit. The crosslinking site monomer is a monomer that can be copolymerized with (meth)acrylate, vinyl carboxylate and ethylene and has a crosslinking group that forms a crosslinking site (also called a crosslinking point). The crosslinking site monomer has a polymerizable carbon-carbon double bond, for example, an acryloyl group, a methacryloyl group, an allyl group, a methallyl group, a vinyl group, or an alkenylene group. As crosslinking groups, for example, carboxyl groups, epoxy groups and active chlorine groups can be cited. The crosslinking site monomer may have one or more of these crosslinking groups. In this specification, monomers that can belong to either (meth)acrylate and vinyl carboxylate and the crosslinking site monomer are classified as crosslinking site monomers.

[0051] Examples of the crosslinking site monomer having a carboxyl group as a crosslinking group include acrylic acid, methacrylic acid, crotonic acid, 2-pentenoic acid, maleic acid, fumaric acid, itaconic acid, and maleic acid monoalkyl esters.

[0052] Examples of the crosslinking site monomer having an epoxy group as a crosslinking group include glycidyl acrylate, glycidyl methacrylate, allyl glycidyl ether, and methallyl glycidyl ether.

[0053] Examples of the crosslinking site monomer having an active chlorine group as a crosslinking group include 2-chloroethyl vinyl ether, 2-chloroethyl acrylate, vinylbenzyl chloride, vinyl chloroacetate, and allyl chloroacetate.

[0054] The content of the crosslinking site monomer may be 0.5 mass % or more, 1 mass % or more, or 1.5 mass % or less, and may be 10 mass % or less, 7 mass % or less, or 4 mass % or less, based on the total amount of monomer units in the acrylic rubber.

[0055] The acrylic rubber may contain other monomers copolymerizable with the above monomers as monomer units. As other monomers, for example, alkyl vinyl ketone, vinyl ether, allyl ether, vinyl aromatic compound, vinyl nitrile, dialkyl maleate, dialkyl fumarate, dialkyl itaconate, dialkyl citrate, dialkyl mesaconic acid, dialkyl 2-pentaconedioate, and dialkyl acetylenedicarboxylate can be cited.

[0056] The acrylic rubber can be obtained by copolymerizing the above-mentioned monomers by a known polymerization method such as emulsion polymerization, suspension polymerization, solution polymerization, bulk polymerization, etc., preferably by suspension polymerization.

[0057] In the above copolymerization, a monomer mixture containing all monomers constituting acrylic rubber may be prepared, and ethylene may be pressed into the monomer mixture to initiate polymerization. In the above copolymerization, vinyl carboxylate may be added in batches, and vinyl carboxylate and (meth)acrylate may be added in batches.

[0058] Among the above copolymerizations, it is more preferable to carry out the polymerization by the following method.

[0059] First, a monomer mixture containing a portion of the used carboxylic acid vinyl ester is prepared, and the remaining portion of the used carboxylic acid vinyl ester or its solution and all the used (meth) acrylic acid ester or its solution are separately prepared. Then, ethylene is pressed into the above-mentioned monomer mixture to initiate polymerization, and while initiating polymerization, the above-mentioned remaining portion of the carboxylic acid vinyl ester or its solution and all the above-mentioned (meth) acrylic acid ester or its solution are added to the system at a constant rate until the polymerization is completed. At this time, the mass ratio of the part of the carboxylic acid vinyl ester used in the above-mentioned monomer mixture to the above-mentioned remaining portion of the carboxylic acid vinyl ester (a part of the carboxylic acid vinyl ester: the remaining portion of the carboxylic acid vinyl ester) is preferably 10:90 to 70:30, and more preferably 20:80 to 60:40.

[0060] Another embodiment of the present invention is a rubber composition containing the above-mentioned acrylic rubber. The rubber composition may further contain a crosslinking agent. The rubber composition may further contain a crosslinking accelerator. In this case, a crosslinked product can be obtained by mixing the rubber composition at a temperature below the crosslinking temperature and heating it at a specified crosslinking temperature. Another embodiment of the present invention is a crosslinked product of the above-mentioned rubber composition.

[0061] The heating conditions during crosslinking can be appropriately set according to the type of the rubber composition and the crosslinking agent. The heating temperature can be 100°C or more, or 200°C or less. The heating time can be 1 hour or more, or 10 hours or less. As the heating method, methods used in crosslinking of rubber such as hot press heating, steam heating, and oven heating can be used.

[0062] The apparatus for kneading, molding, and crosslinking the rubber composition and the apparatus for kneading and molding the crosslinked product of the rubber composition can use the apparatus commonly used for the rubber composition. As the kneading apparatus, a roller, a kneader, a Banbury mixer, a sealed mixer, a twin-screw extruder, etc. can be used.

[0063] The crosslinking agent is not particularly limited as long as it is a crosslinking agent commonly used for crosslinking acrylic rubber. Examples of the crosslinking agent include amine compounds, imidazole compounds, organic peroxides, and polyfunctional (meth)acrylate compounds having two or more (meth)acryloyl groups.

[0064] The content of the crosslinking agent in the rubber composition may be 0.1 parts by mass or more, 0.5 parts by mass or more, or 1 part by mass or more, and may be 15 parts by mass or less, 10 parts by mass or less, or 5 parts by mass or less, based on 100 parts by mass of the acrylic rubber.

[0065] The crosslinking accelerator is not particularly limited as long as it is a crosslinking accelerator commonly used in crosslinking of acrylic rubber. Examples of the crosslinking accelerator include thiazine compounds, quaternary onium salts, guanidine compounds, tertiary phosphine compounds, alkali metal salts of weak acids, and diazabicycloolefin compounds.

[0066] The content of the crosslinking accelerator may be 0.1 parts by mass or more, 0.2 parts by mass or more, or 0.3 parts by mass or more, and may be 5 parts by mass or less, 4 parts by mass or less, or 3 parts by mass or less, based on 100 parts by mass of the acrylic rubber.

[0067] The rubber composition may further contain a filler (reinforcer). As the filler, for example, carbon black, silica, talc and calcium carbonate may be cited. The content of the filler may be, for example, 30 parts by mass or more, or 100 parts by mass or less relative to 100 parts by mass of the acrylic rubber.

[0068] The rubber composition may further contain a lubricant. Examples of the lubricant include liquid paraffin, stearic acid, stearylamine, fatty acid zinc, fatty acid ester, and silicone. The content of the lubricant may be, for example, 0.1 parts by mass or more, or 10 parts by mass or less, relative to 100 parts by mass of the acrylic rubber.

[0069] The rubber composition may further contain an anti-aging agent. Examples of the anti-aging agent include amine-based anti-aging agents and phenol-based anti-aging agents. The content of the anti-aging agent may be, for example, 0.1 parts by mass or more, or 10 parts by mass or less, relative to 100 parts by mass of the acrylic rubber.

[0070] The rubber composition may further contain a surfactant. Examples of the surfactant include alkyl sulfate esters. The content of the surfactant may be, for example, 0.1 parts by mass or more and 10 parts by mass or less relative to 100 parts by mass of the acrylic rubber.

[0071] The rubber composition may further contain a release agent. Examples of the release agent include phosphate esters. The content of the release agent may be, for example, 0.1 parts by mass or more and 10 parts by mass or less relative to 100 parts by mass of the acrylic rubber.

[0072] The rubber composition is suitable for use as a rubber composition for a seal (also referred to as a seal component) or a hose (also referred to as a hose component). In addition, the rubber composition can also be used as a rubber composition for an anti-vibration rubber (also referred to as an anti-vibration rubber component) or a wire-coated rubber (also referred to as a wire-coated rubber component). The crosslinked product of the rubber composition is suitable for use as a seal or a hose. That is, another embodiment of the present invention is a seal or a hose comprising the crosslinked product. In addition, the crosslinked product can also be used as an anti-vibration rubber or a wire-coated rubber. That is, another embodiment of the present invention is an anti-vibration rubber comprising the crosslinked product. As a hose (hose component), a rubber hose, etc. can be cited. As a seal (sealing component), for example, a gasket, a sealing ring, etc. can be cited. These parts can be composed only of a crosslinked product of the rubber composition, or they can be provided with the crosslinked product and other parts.

[0073] Specific examples of hoses (hose components) include transmission oil cooler hoses, engine oil cooler hoses, air duct hoses, turbocharger intercooler hoses, hot air hoses, radiator hoses, power steering hoses, fuel system hoses, and drainage system hoses for automobiles, construction machinery, hydraulic equipment, etc. The hose component may have a reinforcing thread or thread in the middle layer or the outermost layer of the hose.

[0074] Specific examples of seals (sealing parts) include engine cover gaskets, oil pan gaskets, oil seals, lip seals, O-rings, transmission seals, crankshafts, camshaft seals, valve stems, power steering seals, belt cover seals, constant velocity joint dust cover materials, and rack and pinion dust cover materials.

[0075] Specific examples of the vibration-proof rubber (vibration-proof rubber member) include a damping pulley, a center support pad, a suspension bushing, and the like.

[0076] Example

[0077] Hereinafter, the present invention will be described in more detail based on Examples, but the present invention is not limited to the Examples.

[0078] [Example 1]

[0079] (Manufacture of acrylic rubber)

[0080] 5 kg of vinyl acetate, 12 kg of a 5% aqueous solution of partially saponified polyvinyl alcohol (polyvinyl alcohol saponification degree: 88 mol%) and 31 g of sodium acetate were added to a pressure-resistant reaction vessel with an internal volume of 40 liters, and mixed thoroughly in advance with a stirrer to form a uniform suspension. In addition, 5 kg of vinyl acetate and 3 kg of alkyl acrylate (ethyl acrylate, butyl acrylate) were prepared separately. After the air in the upper part of the container was replaced with nitrogen, ethylene was pressed into the upper part of the container at a pressure of 5 MPa. Stirring was continued, and after the container was maintained at 55°C, 2 liters of tert-butyl hydroperoxide aqueous solution (1.6% by mass) were added to initiate polymerization. While initiating the polymerization, the vinyl acetate prepared separately and the alkyl acrylate prepared separately were added at a constant rate until the reaction was completed. The temperature in the container was maintained at 55°C, and the reaction was terminated after 6 hours. 7 liters of sodium borate aqueous solution (3.5% by mass) were added to the obtained copolymer to solidify the copolymer. Next, the solidified copolymer was washed with water, dehydrated and dried to obtain acrylic rubber.

[0081] The monomer composition of the obtained acrylic rubber is 1 H-NMR (AVANCE III HD500 manufactured by Bruker Corporation) was used to measure a solution in which acrylic rubber was dissolved in chloroform.

[0082] (Preparation of rubber composition)

[0083] Next, 100 parts by mass of the obtained acrylic rubber, 50 parts by mass of a filler (carbon black, “SEASTSO” manufactured by Tokai Carbon Co., Ltd.), 1 part by mass of a lubricant A (stearic acid, “LUNAC S-90” manufactured by Kao Corporation), 0.5 parts by mass of an amine antioxidant (NOCRAC CD manufactured by Ouchi Shinko Chemical Industry Co., Ltd.), 0.5 parts by mass of a mold release agent (fatty acid, fatty acid ester, amine, wetting agent mixture, “MOLD WIZ INT-21G” manufactured by Axel Co., Ltd.), 8 parts by mass of a crosslinking agent A (“PERBUTYL P-40” manufactured by NOF Corporation), 3 parts by mass of a crosslinking agent B (trimethylolpropane trimethacrylate, “Monocizer TD-1500” manufactured by DIC Corporation), and 0.5 parts by mass of a crosslinking accelerator A (phenothiazine, manufactured by Seiko Chemical Co., Ltd.) were kneaded on an 8-inch open roll mill to obtain a rubber composition.

[0084] (Production of cross-linked products)

[0085] The obtained rubber composition was cut into sheets with a thickness of 2.4 mm, and then heated and pressurized at 170° C. and a pressure of 4 MPa for 20 minutes using a press vulcanizer to obtain a primary crosslinked product of the rubber composition. Subsequently, the primary crosslinked product was heated at 170° C. for 4 hours in a gear oven to obtain a secondary crosslinked product of the rubber composition.

[0086] (Determination of tensile strength)

[0087] The tensile strength of the obtained secondary crosslinked product was measured in accordance with JIS K6251:2010.

[0088] (Evaluation of oil resistance)

[0089] Using the obtained primary crosslinked product, an evaluation sample was prepared according to JIS K6250:2019, and then immersed in IRM903 oil at 150°C for 72 hours according to JIS K6258:2016. The volume change rate ΔV (%) of the evaluation sample before and after the immersion was measured.

[0090] [Examples 2 to 4]

[0091] The production of acrylic rubber, the preparation of rubber composition and the preparation of crosslinked products were carried out in the same manner as in Example 1 except that the monomer composition was changed as shown in Table 1. The monomer composition of the obtained acrylic rubber was calculated in the same manner as in Example 1. The tensile strength of the obtained secondary crosslinked product was measured in the same manner as in Example 1. In addition, the oil resistance of the obtained primary crosslinked product was evaluated in the same manner as in Example 1.

[0092] [Example 5]

[0093] An acrylic rubber was produced in the same manner as in Example 1 except that the monomer composition was changed as shown in Table 1. The monomer composition was calculated in the same manner as in Example 1.

[0094] Next, 100 parts by mass of the obtained acrylic rubber, 50 parts by mass of a filler (carbon black, “SEASTSO” manufactured by Tokai Carbon Co., Ltd.), 1 part by mass of a lubricant A (stearic acid, “LUNAC S-90” manufactured by Kao Corporation), 0.5 parts by mass of an amine antioxidant (NOCRAC CD manufactured by Ouchi Shinko Chemical Co., Ltd.), 0.5 parts by mass of a mold release agent (fatty acid, fatty acid ester, amine, wetting agent mixture, “MOLD WIZ INT-21G” manufactured by Axel Co., Ltd.), 1.7 parts by mass of a crosslinking agent C (1-(2-cyanoethyl)-2-methylimidazole, “CN-25” manufactured by Shikoku Chemical Industry Co., Ltd.), 0.3 parts by mass of a crosslinking accelerator B (ammonium benzoate, “VULNOC AB” manufactured by Ouchi Shinko Co., Ltd.), and 1 part by mass of a surfactant (sodium dodecyl sulfate, “EMAL 0” manufactured by Kao Corporation) were kneaded on an 8-inch open roll mill to obtain a rubber composition.

[0095] Using the obtained rubber composition, a cross-linked product was prepared in the same manner as in Example 1. The tensile strength and oil resistance of the obtained secondary cross-linked product were measured in the same manner as in Example 1.

[0096] [Comparative Example 1]

[0097] As the acrylic rubber, a commercially available acrylic rubber ("VAMAC DP" manufactured by Du Pont) was used. The acrylic rubber contains an alkyl acrylate and ethylene and does not contain vinyl carboxylate.

[0098] Next, 100 parts by mass of the acrylic rubber, 40 parts by mass of a filler (carbon black, “SEAST SO” manufactured by Tokai Carbon Co., Ltd.), 1 part by mass of a lubricant A (stearic acid, “LUNAC S-90” manufactured by Kao Corporation), 0.5 parts by mass of an amine antioxidant (NOCRAC CD manufactured by Ouchi Shinko Chemical Industry Co., Ltd.), 0.5 parts by mass of a mold release agent (fatty acid, fatty acid ester, amine, wetting agent mixture, “MOLD WIZ INT-21G” manufactured by Axel Co., Ltd.), 0.3 parts by mass of a lubricant B (stearylamine, “FARMIN 80S” manufactured by Kao Corporation), 8 parts by mass of a crosslinking agent A (“PERBUTYL P-40” manufactured by NOF Corporation), and 3 parts by mass of a crosslinking agent B (trimethylolpropane trimethacrylate, “Monocizer TD-1500” manufactured by DIC Corporation) were kneaded on an 8-inch open roll mill to obtain a rubber composition.

[0099] Using the obtained rubber composition, a cross-linked product was prepared in the same manner as in Example 1. The tensile strength of the obtained secondary cross-linked product was measured in the same manner as in Example 1. In addition, the oil resistance of the obtained primary cross-linked product was evaluated in the same manner as in Example 1.

[0100] [Comparative Example 2]

[0101] Into a pressure-resistant reaction vessel with an internal volume of 40 liters, 13 kg of a monomer mixture containing the monomers shown in Table 1 (excluding ethylene), 12 kg of a 5% by mass aqueous solution of partially saponified polyvinyl alcohol (polyvinyl alcohol saponification degree: 88 mol%) and 31 g of sodium acetate were added, and mixed thoroughly in advance using a stirrer to form a uniform suspension. After replacing the air in the upper part of the container with nitrogen, ethylene was pressed into the upper part of the container at a pressure of 5 MPa. Stirring was continued, and after maintaining the container at 55°C, 2 liters of tert-butyl hydroperoxide aqueous solution (1.6% by mass) were added to initiate polymerization. The temperature in the container was maintained at 55°C, and the reaction was terminated after 6 hours. 7 liters of sodium borate aqueous solution (3.5% by mass) were added to the obtained copolymer to solidify the copolymer. Then, the solidified copolymer was washed with water, dehydrated and dried to obtain acrylic rubber. For the obtained acrylic rubber, the monomer composition was calculated in the same way as in Example 1. It should be noted that the composition of the monomers is shown in Table 1.

[0102] Next, 100 parts by mass of the obtained acrylic rubber, 50 parts by mass of a filler (carbon black, “SEASTSO” manufactured by Tokai Carbon Co., Ltd.), 1 part by mass of a lubricant A (stearic acid, “LUNAC S-90” manufactured by Kao Corporation), 0.5 parts by mass of an amine antioxidant (NOCRAC CD manufactured by Ouchi Shinko Chemical Co., Ltd.), 1.4 parts by mass of a crosslinking agent C (1-(2-cyanoethyl)-2-methylimidazole, “CN-25” manufactured by Shikoku Chemical Industry Co., Ltd.), 0.3 parts by mass of a crosslinking accelerator B (ammonium benzoate, “VULNOCAB” manufactured by Ouchi Shinko Co., Ltd.) and 1 part by mass of a surfactant (sodium dodecyl sulfate, “EMAL0” manufactured by Kao Corporation) were kneaded on an 8-inch open roll mill to obtain a rubber composition.

[0103] Using the obtained rubber composition, a cross-linked product was prepared in the same manner as in Example 1. The tensile strength and oil resistance of the obtained secondary cross-linked product were measured in the same manner as in Example 1.

[0104] [Comparative Example 3]

[0105] An acrylic rubber was produced in the same manner as in Comparative Example 2 except that the monomer composition was changed as shown in Table 1. The monomer composition of the obtained acrylic rubber was calculated in the same manner as in Example 1.

[0106] Next, 100 parts by mass of the obtained acrylic rubber, 60 parts by mass of a filler (carbon black, "SEASTSO" manufactured by Tokai Carbon Co., Ltd.), 1 part by mass of a lubricant A (stearic acid, "LUNAC S-90" manufactured by Kao Corporation), 1 part by mass of an amine antioxidant (NOCRAC CD manufactured by Ouchi Shinko Chemical Industry Co., Ltd.), 0.3 parts by mass of a lubricant B (stearylamine, "FARMIN 80S" manufactured by Kao Corporation), 0.4 parts by mass of a crosslinking agent D (hexamethylenediamine urethane, "VC-1" manufactured by Viton Co., Ltd.), and 1.2 parts by mass of a crosslinking accelerator C (a synthetic mixture of an active amine and a retarder, "Rhenogran XLA-60" manufactured by LANXESS Co., Ltd.) were kneaded on an 8-inch open roll mill to obtain a rubber composition.

[0107] Using the obtained rubber composition, a cross-linked product was prepared in the same manner as in Example 1. The tensile strength and oil resistance of the obtained secondary cross-linked product were measured in the same manner as in Example 1.

[0108] Table 1 shows the monomer composition of each acrylic rubber used in Examples and Comparative Examples, and the measurement results of the tensile strength and the evaluation results of the oil resistance in each Example and Comparative Example.

[0109] [Table 1]

[0110]

[0111] The acrylic rubbers of Examples 1 to 5 gave crosslinked products with high tensile strength and excellent oil resistance. On the other hand, the acrylic rubber of Comparative Example 1 gave crosslinked products with high tensile strength, but the oil resistance of the crosslinked products was significantly poor. In addition, the crosslinked products of the acrylic rubbers of Comparative Examples 2 and 3 had low tensile strength.

Claims

1. An acrylic rubber comprising (meth)acrylate, vinyl carboxylate and ethylene as monomer units, Based on the total amount of monomer units in the acrylic rubber, the content of the vinyl carboxylate is 15% by mass or more, and the content of the ethylene is 5% by mass or more.

2. The acrylic rubber according to claim 1, wherein The mass ratio of the ethylene content to the vinyl carboxylate content is 0.1 to 0.

5.

3. The acrylic rubber according to claim 1 or 2, wherein The (meth)acrylate includes at least one selected from the group consisting of alkyl (meth)acrylates and alkoxy (meth)acrylates.

4. The acrylic rubber according to claim 1 or 2, wherein The carboxylic acid vinyl ester includes at least one selected from the group consisting of vinyl acetate, vinyl neodecanoate and vinyl laurate.

5. A rubber composition comprising the acrylic rubber according to claim 1 or 2 and a crosslinking agent.

6. A cross-linked product of the rubber composition according to claim 5.

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